1. Field of the Invention
The present invention relates to an optical switch which can be used in an optical communication system.
2. Description of the Related Art
Hitherto, there is known a waveguide type optical switch comprising a LiNbO.sub.3 crystal which has a comparatively large electro-optical effect. A refractive index profile in the waveguide of such optical switch is controlled by applying a voltage between electrodes provided near the waveguide to change an optical path, and such optical switch enables high speed switching in a nano second order.
As a waveguide type optical switch using quartz glass, one utilizing a heat-optical effect is known. This type of the optical switch comprises a thin film heater on the waveguide and utilizes change of a refractive index due to temperature change caused by heating with the thin film heater to effect switching, whereby a response speed in a millisecond order is achieved.
In addition to the above optical switch, optical switches utilizing an acoustic-optical effect, a magneto-optical effect or an optical Karr effect are proposed.
Since the above optical switches are excellent in high speed response, they are promising as an essential element of an optical switchboard. But, they have following problems.
The above optical switches utilize the change of refractive index of a waveguide material caused by the application of an external energy or force such as electricity, heat, elastic wave, magnetism or light onto the waveguide material. The application of external energy or force corresponds to excitation of an energy in the material. Therefore, it is said that each of the above optical switches utilizes a difference between a ground state and an excited state of the material. However, to maintain the excited state of a physical property of the material, the external energy should be applied to the material constantly during a time period in which the excited state should be maintained. That is, to maintain one of the switching states of the above optical switch, the constant external energy or force should be applied to the material.
In addition, sine a chemical structure of the material in the excited state is unstable, it is difficult to maintain the excited state, and a life of the material is greatly shortened in the excited state. Accordingly, the switch which should maintain the one switching state for a long time suffers from unstability for maintaining the excited state.